Filters are commonly utilized in the processing of electrical signals. For example, in communications applications, such as microwave applications, it is desirable to filter out the smallest possible passband and thereby enable dividing a fixed frequency spectrum into the largest possible number of bands.
Historically, filters have fallen into three broad categories. First, lumped element filters utilize separately fabricated air wound inductors and parallel plate capacitors, wired together to form a filter circuit. These conventional components are relatively small compared to the wave length, and thus provide a compact filter. However, the use of separate elements has proved to be difficult to manufacture, resulting in large circuit to circuit variations. The second conventional filter structure utilizes three-dimensional distributed element components. These physical elements are sizeable compared to the wavelength. Coupled bars or rods are used to form transmission line networks which are arranged as a filter circuit. Ordinarily, the length of the bars or rods is ¼ or ½ of the wavelength at the center frequency of the filter. Accordingly, the bars or rods can become quite sizeable, often being several inches long, resulting in filters over a foot in length. Third, printed distributed element filters have been used. Generally, they comprise a single layer of metal traces printed on an insulating substrate, with a ground plane on the back of the substrate. The traces are arranged as transmission line networks to make a filter. Again, the size of these filters can become quite large. These filters also suffer from various responses at multiples of the center frequency.
Prior art filters have historically been fabricated using normal, that is, non-superconducting materials. These materials have an inherent high loss, and the circuits formed therefrom possess varying degrees of loss. For resonant circuits, the loss is particularly critical. The Q of a device is a measure of its power dissipation or loss. Resonant circuits fabricated from normal metals in a microstrip or stripline configuration have Qs on the order of four hundred. See, e.g., F. J. Winters, et al., “High Dielectric Constant Strip Line Band Pass Filters,” IEEE Transactions On Microwave Theory and Techniques, Vol. 39, No. 12, December 1991, pp. 2182-87.
Microwave properties of high temperature superconductors (HTSCs) have improved substantially since their discovery, and various filter structures and resonators have been formed from HTSCs. See U.S. Pat. No. 5,616,538 to Hey-Shipton, et al. In many applications keeping filter structures to a minimum size is very important. This is particularly true of HTSC filters where the available size of usable substrates is generally limited. In the case of narrow-band microstrip filters (e.g., bandwidths of approximately 2 percent) this size problem may become quite severe.
FIG. 1 is an illustration of a prior art hairpin-resonator bandpass filter 10. See, M. Sagawa, et al., “Miniaturized Hairpin Resonator Filters and Their Application to Receiver Front-End MIC's,” IEEE Trans. MTT, vol. 37, pp. 1991-1997 (December 1989). With reference to FIG. 1, the filter 10 may be thought of as an alternative version of the parallel coupled-resonator filter introduced by S. B. Cohn in “Parallel-Coupled Transmission-Line-Resonator Filters,” IRE Trans. PGMTT, vol. MTT-6, pp. 223-231 (April 1958), except that the individual resonators 12 are folded back upon themselves. The orientations of the hairpin-resonators 12 may alternate (i.e., neighboring resonators face opposite directions) or the orientations of the hairpin-resonators 12 may be substantially similar (i.e., neighboring resonators face in similar directions). Additional resonators 12 may be provided to either side of the filter as represented by an ellipsis. The alternate orientation results in a strong coupling making this structure capable of considerable bandwidth. However, in the case of narrow-band filters, particularly for microstrip filters on a high-dielectric substrate, this structure is undesirable as it may require quite large spacings between the resonators 12 to achieve a desired narrow bandwidth.
FIG. 2 is a graph of a frequency response of the prior art hairpin-resonator filter of FIG. 1 having a passband of 10.44 GHz to 11.82 GHz. With reference to FIG. 2, The measured minimum loss in the passband was approximately −10.576 dB at 10.44 GHz and −9.869 dB at 11.82 GHz.
FIG. 3 is an illustration of another prior art hairpin-resonator filter 30. See, U.S. Pat. No. 5,055,809 to Sagawa, et al. and M. Sagawa, “Miniaturized Hairpin Resonator Filters and Their Application to Receiver Front-End MIC's,” IEEE Trans. MTT, vol. 37, pp. 1991-1997 (December 1989). With reference to FIG. 3, the open-circuited ends 34 of the plural resonators 32 are considerably foreshortened and a capacitive gap 36 is provided to bring the remaining structure into resonance. The resonators 32 are then semi-lumped, with the lower portion 38 being inductive and the upper portion 39 being capacitive. The coupling between resonators 32 is almost entirely inductive, and it makes little difference whether adjacent resonators are inverted with respect to each other or not. Additional resonators 32 may be provided to either side of the filter as represented by an ellipsis. As illustrated in FIG. 3, the resonators 32 may possess the same orientation. If the resonators have sufficiently large capacitive loading, these resonator structures can be quite small, but, typically, their Q is inferior to that of a full hairpin resonator. Also, there will normally be no resonance effect in the region between the resonators so that the coupling mechanism cannot be used to generate poles of attenuation beside the passband in order to enhance the stopband attenuation.
Therefore, a need exists for compact, reliable, and efficient narrow-band filters possessing very high Q resonators. Despite the clear desirability of improved electrical circuits, including the known desirability of converting circuitry to include superconducting elements, room remains for improvement in devising alternate structures for filters. It has proved to be especially difficult to substitute HTSC in conventional circuits to form superconducting circuits without severely degrading the intrinsic Q of the superconducting films. Among the problems encountered are radiative losses and tuning, which remain despite the clear desirability of improved filters. As is described above, size has also remained a concern, especially for narrow-band filters. Also, power limitations arise in certain structures. Despite the clear desirability for forming microwave filters for narrow-band applications, to permit efficient use of the frequency spectrum, a need remains for improved designs capable of achieving those results in an efficient and cost effective manner.
Accordingly, there is a need for a method and apparatus for a novel hairpin microstrip bandpass resonator that would overcome the deficiencies of the prior art. Therefore, an embodiment of the present subject matter provides a microstrip filter having a plurality of hairpin microstrip resonators each having two substantially rectangular legs connected at one end and generally configured in a “U” shape. The microstrip filter comprises a plurality of resonators, a first resonator operatively connected to a first feed point and a second resonator operatively connected to a second feed point. A third of the plural resonators is operatively connected between the first and second resonators where an end portion of one of the legs of the resonators is tapered so that a thickness of the leg is greater at one end of the leg than at another end of the leg. The apparatus may further comprise a second plurality of resonators in place of the third resonator.
In another embodiment of the present subject matter an end portion of one of the legs of the third resonator may be tapered so that a thickness of a leg is greater at one end of the leg than at another end of the leg. An alternative embodiment of the present subject matter provides an end portion of one of the legs of the first resonator may tapered so that a thickness of the leg is greater at one end of the leg than at another end of the leg. In yet another embodiment, legs of the third and first resonators may also be tapered.
In yet another embodiment of the present subject matter a method is provided for increasing the operational bandwidth of a microstrip filter having a plurality of hairpin microstrip resonators each having two substantially rectangular legs connected at one end and generally configured in a “U” shape. The method comprises the steps of providing a first of the plural resonators operatively connected to a first feed point and providing a second of the plural resonators operatively connected to a second feed point. The method further comprises the steps of increasing a thickness of a portion of one leg of a third of the plural resonators such that a thickness of the one leg is greater at one end of the one leg than at another end of the one leg, and operatively connecting the third resonator between the first and second resonators. An alternative embodiment may interleave the legs of adjacent resonators and/or may substitute a second plurality of resonators for the third resonator.
In yet a further embodiment of the present subject matter, a microstrip filter is provided having a plurality of hairpin microstrip resonators each having two substantially rectangular legs connected at one end and generally configured in a “U” shape. The microstrip filter comprises a first of the plural resonators operatively connected to a first feed point, a second of the plural resonators operatively connected to a second feed point, and a third of the plural resonators operatively connected between the first and second resonators wherein the length of one of the legs of the third resonator is different than the length of one of the legs of the first or second resonators. An end portion of one of the legs of the plural resonators may also be tapered so that a thickness of the leg is greater at one end than at another end of the leg. Alternative embodiments of the filter may provide legs of the third resonator having a first length and the legs of the first or second resonators having a second length wherein the first and second lengths are not equal, and may substitute a second plurality of resonators for the third resonator.
Another embodiment of the present subject matter provides a method for shifting the center frequency of a microstrip filter having a plurality of hairpin microstrip resonators each having two substantially rectangular legs connected at one end and generally configured in a “U” shape. The method comprises the steps of providing a first of the plural resonators operatively connected to a first feed point, providing a second of the plural resonators operatively connected to a second feed point, changing the length of at least one of the legs of a third of the plural resonators, and operatively connecting the third resonator between said first and second resonators. An alternative method provides that the third resonator may further comprise a second plurality of resonators.
In yet another embodiment of the present subject matter, a microstrip filter is provided having a plurality of hairpin microstrip resonators each having two substantially rectangular legs connected at one end and generally configured in a “U” shape. The microstrip filter comprises a first of the plural resonators operatively connected to a first feed point, a second of the plural resonators operatively connected to a second feed point, and a third of the plural resonators operatively connected between the first and second resonators, where adjacent legs of adjacent plural resonators may be interleaved. A further embodiment may taper the legs of any number of the plural resonators.
An additional embodiment of the present subject matter provides a method for increasing the return loss of a microstrip filter having a plurality of hairpin microstrip resonators each having two substantially rectangular legs connected at one end and generally configured in a “U” shape. The method comprises the steps of operatively connecting a first of the plural resonators to a first feed point, providing a second of the plural resonators operatively connected to a second feed point, operatively connecting a third of the plural resonators between the first and second resonators, and interleaving adjacent legs of adjacent plural resonators. The method may also comprise the step of increasing a thickness of a portion of any of the legs of the plural resonators. The method may further comprise the step of maintaining a substantially constant distance between adjacent legs. An alternative embodiment may substitute a second plurality of resonators for the third resonator.
These embodiments and many other objects and advantages thereof will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the embodiments.